The present invention relates to a multi-part, typically, two-part cold curing composition for use in metallography to produce metallography mounts and, in particular but not exclusively to one which utilises cyclic ester side group containing acrylic monomers.
Metallographic study, or metallography, also known as metallographic encapsulation is the examination of macro- and microstructural features on prepared surfaces of materials. Specimens to be examined by metallography are typically prepared by firstly encapsulating each specimen within a plastic mount to facilitate handling during sample preparation. This is particularly important for specimens that need perfect edge retention or protection of layers. Mount sizes typically range from about 25 mm to 75 mm in diameter. After mounting in resin, sample preparation then consists of grinding and then polishing using successively finer abrasives to obtain the desired surface finish. For microstructure examination, a mirror finish is needed, but a finely-ground finish is adequate for macrostructure evaluation. Chemical or other etching methods are often used to delineate macro- and microstructural features. Once prepared, samples are examined by the unaided eye, light microscopy, and/or electron microscopy.
Two types of mounting techniques are used-hot compression mounting (also called hot mounting) and cold mounting. The choice of mounting technique to use depends on several factors, such as the number of samples to examine, the quality required and cost.
Hot mounting requires the use of a metallurgical mounting press. It produces hard mounts for encapsulating metallurgical samples in short cycle times and is ideal for a regular succession of incoming samples. The resulting mounts are high quality, uniform size and shape, and require a short process time.
Cold mounting is suitable for a large series of samples coming to the lab simultaneously, and also for single samples. Cold mounting is especially suited for mounting specimens that are sensitive to heat or pressure. Additionally, cold mounting does not require an investment in a mounting press and is therefore good for infrequent mounting tasks. Some cold mounting resins can be used for vacuum impregnation.
The three main types of resins used in cold mounting are epoxy resins, acrylic resins and polyester resins. All are supplied as two part systems. Epoxy and polyester systems involve resin and hardener. Acrylic systems utilize acrylic resin powder and liquid monomer. Resin curing involves an exothermic reaction and it is important to mix the correct proportions of constituents to produce mounts of high quality.
Acrylic systems have the particular advantages of being easy to use and provide short curing times. They are well-suited for both serial mounting of irregularly shaped specimens and for routine work or single specimens.
It is the field of acrylic resins and monomers for cold mounting that this invention is concerned with. The cold mounting process involves the cold curing (also known as self-curing or autopolymerising) of the liquid monomer when combined with acrylic resin powder at ambient temperature.
Acrylic resins for metallography applications are well known. The solid part generally comprises a homopolymer or copolymer of either methyl methacrylate (MMA), ethyl methacrylate (EMA), iso-butyl methacrylate (iBMA) or n-butyl methacrylate (nBMA).
The liquid part generally comprises monofunctional methacrylate monomers such as MMA, EMA, iBMA or nBMA either on their own or as a mixture of two or more. Crosslinking monomers may also be present as part of the liquid part, such as ethylene glycol dimethacrylate.
Unfortunately, monofunctional methacrylate monomers such as MMA, EMA, iBMA and nBMA suffer from problems that make them unattractive for use in cold mounting applications. For example, they tend to have a relatively high odour compared to epoxy resins and polyester resins. Additionally, they fall within a class of materials that is rated as flammable and hazardous for transport, thereby necessitating special transport arrangements and storage in particular areas. These factors lead to increased costs and higher operating standards when using such materials.
Odour problems can be reduced by using higher molecular weight monofunctional methacrylate monomers such as 2-ethyl hexylmethacrylate, octylmethacrylate or dodecyl methacrylate, but these have the disadvantage of increasing the time for the solid/liquid combination to polymerise and set. Also, the resultant mount is rather soft due to a combination of reduced glass transition temperature (Tg) and the conversion of monomer to polymer being lower.
Functional monomers such as methacrylic acid or hydroxyalkyl methacrylates such as hydroxyethyl methacrylate or hydroxypropyl methacrylate may be considered. The volatility of these monomers is lower than monofunctional methacrylate monomers such as MMA, EMA, iBMA and nBMA, and they therefore have lower odour, but they bring other problems to the application. For example, methacrylic acid is corrosive (therefore requiring special handling) and it is a solid at temperatures below 15° C. Hydroxyethyl methacrylate and hydroxypropyl methacrylate are potential skin sensitisers.
U.S. Pat. No. 5,080,583 describes a two component acrylic monomer-polymer system for making an orthodontic appliance through a cold cure approach, i.e. by mixing the two components together at ambient temperature and allowing them to harden under the action of initiator and activator or accelerator. A fragrance oil is also added to provide a pleasant taste and smell to the orthodontic appliance. Additionally, metallic hardware is embedded in the acrylic base such that the embedded hardware extends outward to contact the patient's mouth when the device is in position in the patient's mouth. However, U.S. Pat. No. 5,080,583 does not embed the metallic hardware with a view to metallographical study. Additionally, it teaches the use of methyl methacrylate in the liquid part, a monomer that has the problem of volatility, odour and flammability if used for metallography, as detailed above.
EP2502942, discusses use of a single part dual initiator UV and heat cure composition. The document teaches the avoidance of peroxide initiators and the use of azo initiators for heat curing. The single part composition replaces 2-part (2-K) systems which are said to cause a lack of transparency and require high pressure.